Microlithographic projection exposure apparatuses are used to transfer structures contained in a mask or arranged thereon to a photoresist or some other light-sensitive layer. Important optical components of a projection exposure apparatus are a light source, an illumination system which conditions projection light generated by the light source and directs it onto the mask, and a projection objective which images that section of the mask which is illuminated by the illumination system onto the light-sensitive layer.
The shorter the wavelength of the projection light, the smaller the structures that can be defined on the light-sensitive layer with the aid of the projection exposure apparatus. The next generation of projection exposure apparatuses will use projection light in the extreme ultraviolet spectral range (EUV), having a wavelength of 13.5 nm. Such apparatuses are often referred to as EUV projection exposure apparatuses.
However, there are no optical materials which have a sufficiently high transmittance for such short wavelengths. Therefore, in EUV projection exposure apparatuses the lenses and other refractive optical elements that are customary at longer wavelengths are replaced by mirrors, and the mask, too, therefore also contains a pattern of reflective structures.
Providing mirrors for EUV projection exposure apparatuses constitutes a major technological challenge. Coatings which are suitable for EUV light and are applied to a mirror substrate often include more than 30 or 40 double layers having a thickness of just a few nanometers which are vapour-deposited one above another in technologically complex processes. Even with coatings having such a complex construction, the reflectivity of the mirrors for the EUV light is usually hardly more than 70%, and even this applies only to light which impinges on the reflective coating perpendicularly or at angles of incidence of a few degrees.
The consequence of the comparatively low reflectivity of the mirrors is that during the development of projection exposure apparatuses an effort has to be made to use as few mirrors as possible, since each mirror is associated with losses of light and ultimately reduces the throughput of the projection exposure apparatus.
However, the relatively low reflectivity of the mirrors is also accompanied by thermal problems, since that portion of the high-energy EUV light which is not reflected by the coating is absorbed and leads to a temperature increase in the mirrors. The heat generated in the process has to be dissipated substantially by way of thermal conduction via the mirror substrate, since projection exposure apparatuses have to be operated in a vacuum owing to the high absorption of EUV light by gases and heat transfer by convection is therefore ruled out.
In order that temperature gradients occurring in the mirror substrates do not lead to an undesired deformation of the mirrors, it is expedient to use materials for the mirror substrates which have a coefficient of thermal expansion that is as small as possible or even vanishingly small at the operating temperature. Glass-based materials of this type are sold for example by Schott under the trade name Zerodur® and by Corning under the trade name ULE®. By using additional measures, thermal deformations caused by absorption of EUV light can be kept small or at least the effects thereof on the optical properties of the projection objective can be kept within tolerable limits.
U.S. Pat. No. 7,477,355 B2 proposes heating mirrors with the aid of an additional heating mechanism such that the substrate material of the mirrors is at a temperature at which the coefficient of thermal expansion is zero or at least minimal. Temperature fluctuations during the operation of the apparatus then have no effect, or only a small effect, on the imaging properties of the mirror.
U.S. Pat. No. 7,557,902 B2 describes a projection objective in which two mirrors are formed of materials whose coefficient of thermal expansion increases with increasing temperature in one of the two mirrors and decreases with increasing temperature in the other mirror. By selecting suitable mirrors, although the two mirrors deform significantly in the case of a temperature change, the optical effects of these deformations largely cancel one another out.
Similar issues also are to be addressed in the case of facet mirrors, except that in that case generally a carrying body that carries the individual mirror facets is affected by thermally induced deformations. Owing to the comparatively low thermal conductivity of glass-based materials, metals are usually used as carrying body. However, in general, there are no metals which have at an operating temperature a coefficient of thermal expansion having a low magnitude similar to that in the case of the glass-based materials mentioned above.
Thermally induced deformations also exist in projection exposure apparatuses designed for longer wavelengths, e.g. 193 nm (VUV) or 254 nm (DUV). This usually affects lens elements and other refractive optical elements which heat up inhomogeneously as a result of (albeit slight) absorption of projection light and change their form as a result.
DE 103 17 662 A1 discloses an EUV projection exposure apparatus including a heating light source, which illuminates selected regions on imaging mirrors with additional heating light. As a result of absorption of the heating light, an at least approximately homogeneous temperature distribution is established on the surface of the mirror. With a suitable design it can be achieved that at thermal equilibrium a temperature is established at which the coefficient of thermal expansion of the mirror substrate has an absolute value minimum. As a result, generally, relatively small temperature fluctuations or residual inhomogeneities of the temperature distribution can no longer lead to appreciable thermal deformations of the mirror substrate and thus to imaging aberrations.
In order to illuminate only selected regions on the mirror with the heating light, a transmission filter is arranged downstream of the heating light source in this known projection exposure apparatus, the transmission filter shading those regions on the mirror surface on which projection light impinges and which are therefore intended not to be additionally heated by heating light. If the heating light is intended to form a pattern that is as sharply delimited as possible on the mirror surface, an additional imaging optical unit can be provided, which images the transmission filter onto the mirror surface. In another embodiment, a laser is used as heating light, a controllable ray deflecting device being assigned to the laser. By use of the ray deflecting device, the laser ray generated by the laser is directed only onto the desired regions on the mirror surface. In this case, the ray deflecting device can be embodied in a manner similar to that in barcode scanners known per se.
However, when a transmission filter is used, under certain conditions not insignificant, a portion of the heating light is lost in the transmission filter. If only relatively small regions are intended to be heated with high intensity on the mirror surface, then the use of transmission filters usually involves very powerful heating light sources, as a result of which the transmission filter heats up to an even greater extent.
With the use of a relatively fine laser ray which, in a scanner-like mirror, sweeps over the regions on the mirror surface which are to be illuminated by the heating light, then generally no loss of light occurs. However, the intensity of the laser ray is high in order to achieve a sufficiently high heat input, and the sensitive reflective coating of the mirrors can be damaged by high local radiation action under certain circumstances.